The present invention relates, in general, to surveys for locating target subterranean bodies, and more particularly to directional subsurface drilling of off-vertical wells using a magnetometer instrument to survey from a borehold the direction and range to a predetermined subsurface target and provide information for guiding futher drilling.
In drilling an oil or gas well, it is often desirable to drill the hole as nearly as possible in a true vertical course. Realizing that a well cannot be drilled that is exactly vertical, at the conclusion of the drilling of the well, it is routine practice to have a logging survey made in order to determine the deviation off-vertical of the well at various depths. The survey involves, in one case, the operation of an instrument, as it is raised or lowered through the borehole, to register changes in its orientation off-vertical using the earth's magnetic field and gravity as references. In another case, changes with respect to a gyroscopic reference are recorded. Instruments of this type are well known to those skilled in art.
When a well "blows out" or goes out of control, it is desirable to intersect that well at a point above the high pressure producing formation in a suitably permeable zone, so as to allow fluid flow in order to plug the borehole and eliminate the blowout. Such a relief well is drilled in order that cement or some similar material can be pumped down to kill the blowout. In wells having large flow rates, and particularly those which have caught fire as well, it is required that an off-vertical well be drilled to intersect the first well to provide a path to the point where shutoff is desired to be made.
Generally speaking, off-vertical well drilling to intercept a previously drilled well can be done fairly accurately if the location of the target is known with sufficient accuracy. However, due to the lack of accuracy in the logging of the off-vertical deviations of the first well, the exact position of the desired target point along the blown out well is generally not accurately known. Typically, the location will be known only to within about ten to forty feet. In view of the fact that the drill string being used to drill the off-vertical relief well cannot be turned on a sharp radius, and thus must be set up directionally at a point far from the first well, it is difficult to precisely intersect the first well. Several attempts may be required to effect interception. If, however, the target location along the first well site were able to be accurately pin-pointed, drilling could proceed more readily to intersection therewith. This, of course, is generally not the case.
Therefore, to expeditiously drill off-vertical relief wells to intersect a first well in order to shut off a well out of control, it is necessary to employ the technique of directional drilling. Directional drilling involves controlling the course of a borehole by using surface and subsurface instruments to direct the drilling toward a specific target. Direction recording instruments are used to determine the desired direction of drilling with deflecting tools and/or directional methods being used down hole to control the downward course of the well.
One approach to direction recording instruments for use in off-vertical well drilling is a system in which a magnetometer is located in a target well with a magnetic field generator, such as an electromagnet, being located in a second well some distance from the first. The electromagnet is carried by a drill string which is to be guided in accordance with the measurements of the field generated at the target well as obtained by the magnetometer. These measurements provide an indication of the direction of the generated field with the changes in the measured components providing an indication of the direction of travel of the drill with respect to the target magnetometer. This technique of off-vertical well drilling is taught in the prior art by U.S. Pat. Nos. 3,285,350 and 3,406,766 to J. K. Henderson.
Another approach to directional drilling of off-vertical wells is that of U.S. Pat. No. 3,725,777 to Robinson et al. The approach disclosed therein provides a method for locating a previously drilled well which is cased with a material having a remanent magnetization. Magnetometers measure the total strength of the existing magnetic field which is a combination of the magnetized casing plus the earth's field. Possible locations of the previously cased well are calculated; and assuming the strength and direction of the earth's field, the strength and direction of the field contributed by the cased well can be determined. The distance and direction to the cased well are determined by machine calculations involving a least squares fit analysis.
Another approach involving the determination of the distance between a cased well and a directional well is that of U.S. Pat. No. 3,748,574 to Mitchell et al, which discloses a technique using resistivity measurements. In this technique, the expected resistivity of the formations surrounding the off-vertical well is determined in calculations made of the anticipated reduction is resistivity caused by the presence of the casing. A nomogram is prepared by plotting the calculated reduction versus the assumed distances for each calculated formation resistivity. The measured resistivity caused by the casing in the distance between the two wells is then obtained form the nomogram.
Generally, guidance systems for off-vertical well drilling will include subsurface magnetic field direction sensing devices and surface recording instruments for displaying the information concerning the magnetic field being sensed. The subsurface magnetic field direction sensing device is usually some type of magnetometer which detects the direction of emanation of the magnetic field of the target and of the earth, with the outputs therefrom being connected to the surface recording instruments.
Typically, the magnetic field direction sensing device will be a fluxgate magnetometer having a low reluctance magnetically directionally sensitive loop with drive coils and sense coils wound thereon. An oscillator produces AC current flow in the parallel drive coils which developes an alternative magnetic flux in the loop. When the loop is not subject to any ambient magnetic field, the voltage induced in each sense coil will be equal and opposite, so that upon summing of the voltages no output is obtained. When the magnetic loop is subjected to an ambient magnetic field having lines of force including a vector component parallel to the loop, the balance between the sense coils is disturbed and an AC voltage is produced at the output. Since the magnetic field direction sensing device will be sensitive to the earth's magnetic field, some type of neutralizating technique is usually employed to adjust the flux being created in the loop to remove the influence of the earth's field and drive the output voltage of the sense coils to zero. Magnetometers of this type are sensitive only to magnetic fields having a vector component parallel to the core and is, therefore, not sensitive to magnetic fields perpendicular to the length of the loop.
In order to establish the direction of emanation of the magnetic field, it has been usual in prior magnetometer systems to utilize two mutually perpendicular fluxgate magnetometers defining X and Y coordinate vectors of the detected field. The vectors are generally resolved electronically and displayed on some type of surface recording instrument. Typically, the surface recording instrument will serve to resolve the vector components of the sensed magnetic field in a conventional manner using rectangular coordinates, as by plotting the component amplitudes and solving graphically for the actual field direction in the plane of the sensors. Representative of the foregoing described magnetic field sensing devices and magnetometer systems in Schad, U.S. Pat. No. 3,731,752. In this reference, it is further suggested that a third magnetometer could be used to measure X, Y and Z magnetic field components (Col. 4, line 55, et seq.).
Prior magnetometer guidance systems for off-vertical well drilling, such as that described above, position the magnetic field direction sensing device in an existing well that is to be intersected by a second well. Thus, the magnetometer becomes the target with the electromagnet, creating a detectable magnetic field. The requirement that a magnetic field generator be used to set up a detectable magnetic field can present insurmountable problems in those situations, such as a blowout well, wherein it is not possible to place a magnetometer device or a field generating source in the target well.
Thus, it is desirable to have a surveying system for guiding off-vertical well drilling which is capable of locating a subsurface ferromagnetic target such as a length of drill string, a drill tool or well casing in the target well. Such ferromagnetic material will demonstrate and possess remanent magnetization since most drill pipe and well casing is electromagnetically inspected before it is installed, leaving a residual magnetic field in the casing. Even were this not the case, the magnetic influence of the earth's field will induce some magnetization which may be detected in a ferromagnetic material in the target well. It is further desirable to have a surveying system that provides not only the direction of the subsurface target from the borehole, but provides the range to the target also.